1. Field of the Invention
The present invention relates to a semiconductor device, more particularly relates to a semiconductor device having a photo diode for receiving a plurality of light having different wavelengths.
2. Description of the Related Art
Light receiving elements, that is, photodiodes, are being widely used as optical sensors for converting an optical signal to an electrical signal for control use in optical sensor applications in a variety of photoelectronic conversion apparatuses and for optical pickup applications installed in CD, DVD, and other optical disk drives.
There are several kinds of the above photo diode, but basically it is comprised of a pn junction of a semiconductor.
When applying an inverse bias to a pn junction, a depletion layer spreads in a p-type semiconductor region and an n-type semiconductor region, respectively, from a pn-junction surface. Electron-hole pairs are generated by light absorbed mainly in the depletion layer. The electrons move to the n-type semiconductor region drawn by the electric field, while the holes move to the p-type semiconductor region and are detected as a current.
Particularly, by providing an I layer (pxe2x88x92-layer or nxe2x88x92-layer) containing conductive impurities at a low concentration between the p-layer and n-layer, it is possible to have the depletion layer spread by a low voltage. This is called a PIN photo diode.
FIG. 1A is a sectional view of the above PIN photo diode.
For example, a pxe2x88x92-type semiconductor layer 11 is formed on the p+-type semiconductor substrate 10, furthermore, an n-type semiconductor layer 12 is formed at a surface region of the pxe2x88x92-type semiconductor layer 11 so that the pn junction is formed.
A p+-type semiconductor layer 13 is formed to surround the PIN diode region. It becomes a take-out portion of a p-region and isolates the PIN diode region from other elements.
A protective insulating film 14 comprised of silicon oxide etc. is formed at an upper layer of the n-type semiconductor layer 12 and the p+-type semiconductor layer 13. Contacts respectively reaching the n-type semiconductor layer 12 and the p+-type semiconductor layer 13 are opened, and take-out electrodes (15a and 15b) are formed.
When applying a predetermined inverse bias to the pn-junction of the PIN photo diode shown in FIG. 1A, a depletion layer V spreads from the pn-junction surface to the directions of the pxe2x88x92-type semiconductor layer 11 and n-type semiconductor layer 12 as shown in FIG. 1B.
Here, the depletion layer V spreads so that the total number of carriers becomes equal at the n-side and p-side, so spreads more to the pxe2x88x92-type semiconductor layer 11 side having a low carrier concentration.
However, a photo diode such as the above conventional PIN photo diode is normally designed to be in a structure which is optimized for light having a certain wavelength, for example, near 780 nm when used in a CD system and near 650 nm when used in a DVD system. Generally, in a photo diode of the same structure, there is a large wavelength dependence of the light receiving sensitivity, so when trying to receive a plurality of light having different wavelengths such as light of wavelengths of 780 nm and 650 nm, by an identical photo diode or by a plurality of photo diodes of the same structure present on an identical substrate, the sensitivity ends up greatly differing at the different wavelengths. Thus, for practical use, it was necessary to make the sensitivities match in a required wavelength region.
The above wavelength dependence of the light receiving sensitivity is derived from the fact that the light receiving sensitivity differs in accordance with the laser wavelengths and the structure of the photo diode since when the reflection of light at the light receiving surface is 0%, the light receiving sensitivity S=excex/hc at a quantum efficiency of 100% stands, so the light receiving sensitivity at the quantum efficiency of 100% rises as the wavelength becomes longer in proportion to the wavelength, while the length of light absorption becomes shorter as the light wavelength becomes shorter.
An object of the present invention is to provide a semiconductor device having a photo diode having substantially the same sensitivity for a plurality of light of different wavelengths.
To achieve the above object, a semiconductor device of the present invention is a semiconductor device having a photo diode comprising a first conductivity type semiconductor layer and a second conductivity type semiconductor layer formed at a surface layer portion of said first conductivity type semiconductor layer, wherein the sensitivity of said photo diode to light of a first wavelength and the sensitivity to light of a second wavelength which is different from said first wavelength are made to become substantially the same by designing a region in which a depletion layer spreads from a junction surface of said first conductivity type semiconductor layer and said second conductivity type semiconductor layer when inverse biases are applied to said first conductivity type semiconductor layer and said second conductivity type semiconductor layer.
In the semiconductor device of the present invention, preferably the sensitivity of said photo diode to light of a first wavelength and the sensitivity to light of a second wavelength are made to become substantially the same by designing impurity concentrations and said inverse biases of said first conductivity type semiconductor layer and said second conductivity type semiconductor layer and by designing said region in which a depletion layer spreads.
Preferably, in the above semiconductor device of the present invention, the depletion layer is designed to spread in a region including a region 3 to 6 xcexcm or 2 to 7 xcexcm in the depth direction from a surface of the second conductivity type semiconductor layer.
Preferably, in the above semiconductor device of the present invention, the first conductivity type semiconductor layer is formed on a first conductivity type semiconductor substrate containing a first conductivity type impurity at a higher concentration than the first conductivity type semiconductor layer.
More preferably, the concentration of the first conductivity type impurity on the surface of the first conductivity type semiconductor substrate is at least 1xc3x971017/cm3.
More preferably, a distance between an end face of said depletion layer on said first conductivity type semiconductor substrate side and the surface of said first conductivity type semiconductor substrate is 3 xcexcm or less.
In the semiconductor device of the present invention, preferably said first wavelength is 780 nm and said second wavelength is 650 nm.
Furthermore, to achieve the above object, a semiconductor device of the present invention is a semiconductor device comprising a first conductivity type semiconductor substrate, a first conductivity type semiconductor layer formed on a first conductivity type semiconductor substrate and containing a first conductivity type impurity at a lower concentration than said first conductivity type semiconductor substrate, and a second conductivity type semiconductor layer formed at a surface layer portion of said first conductivity type semiconductor layer, wherein a photo diode is formed by spreading a depletion layer from a junction surface of said first conductivity type semiconductor layer and said second conductivity type semiconductor layer when inverse biases are applied to said first conductivity type semiconductor layer and said second conductivity type semiconductor layer, and the concentrations of the impurity of said first and second conductivity layers are adjusted so that the depletion layer spreading region is made wherein the sensitivity of said photo diode to light of a first wavelength and the sensitivity to light of a second wavelength which is different from said first wavelength become substantially the same.
The semiconductor device of the present invention is provided with a photo diode having a first conductivity type semiconductor layer and a second conductivity type semiconductor layer formed at a surface layer portion of said first conductivity type semiconductor layer, wherein the sensitivity to light of a first wavelength and the sensitivity to light of a second wavelength which is different from said first wavelength are made to become substantially the same by designing the impurity concentrations and said inverse biases of said first conductivity type semiconductor layer and said second conductivity type semiconductor layer, by designing a region in which a depletion layer spreads from a junction surface of said first conductivity type semiconductor layer and said second conductivity type semiconductor layer when inverse biases are applied to said first conductivity type semiconductor layer and said second conductivity type semiconductor layer, and, for example, by designing the depletion layer to spread to a region 3 to 6 xcexcm or 2 to 7 xcexcm in the depth direction from a surface of the second conductivity type semiconductor layer.
The semiconductor device having the above photo diode is a semiconductor device having a photo diode designed with a region in which a depletion layer spreads so that the sensitivity of the photo diode to light of a first wavelength and the sensitivity to light of a second wavelength which is different from said first wavelength become substantially the same and therefore having substantially the same extent of sensitivity to a plurality of light of different wavelengths.
The semiconductor device of the present invention may be made a PIN photo diode enabling the depletion layer to easily spread by a low voltage by forming the first conductivity type semiconductor layer on a first conductivity type semiconductor substrate containing a first conductivity type impurity in a higher concentration than the first conductivity type semiconductor layer.
In the above configuration, by making the surface concentration of the first conductivity type impurity of said first conductivity type semiconductor substrate at least 1xc3x971017/cm3 or by making the distance between an end face of said depletion layer at the first conductivity type semiconductor substrate side and the surface of said first conductivity type semiconductor substrate 3 xcexcm or less, it is possible to suppress more than the necessary photoelectric conversion in the region below the depletion layer and improve the frequency characteristics to enable the speed of the device to be increased.